Improvement of low temperature toughness of low-alloy steel forgings by intermediate intercritical quenching

An intermediate intercritical quenching process, introduced between the conventional quenching and tempering treatments, was studied to improve the low-temperature toughness of heavy forgings made of low-alloy steels with C < 0.25%. After a design phase, validation of the proposed new heat treatment was carried out by laboratory tests. The benefits of the Intermediate Intercritical Quenching were evaluated comparing the mechanical properties and microstructures of forged coupons subjected to standard Quenching & Tempering, double Quenching & Tempering and Quenching followed by Intermediate Intercritical Quenching and Tempering. The introduction of the Intermediate Intercritical Quenching treatment, while maintaining the strength levels within the target values, leads to a considerable increase of low-temperature impact toughness for the investigated steels (0.12–0.16%C, 0.7–1.35%Mn, 1.5–2.0%Ni). This behavior was attributed to microstructure refinement through reversed transformation during intercritical annealing. Industrial trials confirmed that tensile strength of 490 to 510 MPa and yield strength of 340 to 380 MPa can be achieved with very high Charpy V-notch (CVN) energy and fracture toughness levels (mean CVN energy at − 40 °C = 354–427 J, KJd = 582–623 MPa m 0.5), even in case of large thickness (up to 350 mm).